Butyl rubber is extremely low in unsaturation degree, and therefore, is excellent in weatherability, heat resistance, ozone resistance, etc. and is also low in air permeability. Thus, butyl rubber is suitable for use as a sealant, adhesive, etc. Therefore, as methods for cross-linking the butyl rubber, sulfur cross-linking, quinoid cross-linking, resin cross-linking, etc. are known, but currently it is hard to say that any method is satisfactory in practice. That is, sulfur cross-linking requires cross-linking at a high temperature over a long time. Further, quinoid cross-linking usually uses toxic red lead oxide as an oxidizing agent to activate the quinoid, and therefore has problems in terms of environment safety. Further, resin cross-linking has a remarkably slow reaction rate and requires heating at a high temperature over a long time. Thus, the product is liable to be in a state not completely cross-linked, therefore there is the problem that, during use, a cross-linking reaction is liable to proceed and the physical properties are liable to greatly change. Further, among the methods of cross-linking a diene-based rubber etc., cross-linking by organic peroxide, which is extremely excellent in heat resistance, is almost never used as a method for cross-linking butyl rubber. This is because, if this cross-linking is applied to butyl rubber, the decomposition reaction of the main chain is liable to proceed with priority and the resultant product is liable to be softened rather than the cross-linking. On the other hand, partially cross-linked butyl rubber is commercially available as a butyl rubber capable of being subjected to peroxide cross-linking, but this has the problem of insufficient processability. Further, Japanese Patent Publication (A) No. 6-172547 discloses a method of cross-linking uncross-linked butyl rubber in the presence of an organic peroxide and a polyfunctional monomer containing an electron attractive group, but, according to this method, severe reversion is liable to occur.